/Engine/Plugins/MovieScene/SequencerScripting/Source/SequencerScriptingEditor/Public/SequencerTools.h
Returns if Render to Movie is currently in progress.
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintCallable, Category="Editor Scripting | Sequencer Tools | Movie Rendering", Meta=(DeprecatedFunction)) static bool IsRenderingMovie()